Airborne Wind Lidar measurements of vertical and horizontal winds for the investigation of orographically induced gravity waves

Abstract

irborne coherent Doppler wind lidar measurements, acquired during the GW-LCYCLE I campaign performed from 2 to 14 December 2013 in Kiruna, Sweden, are used to investigate internal gravity waves (GWs) induced by flow across the Scandinavian Alps. Vertical wind speed is derived from lidar measurements with a mean bias of less than 0.05 ms?1 and a standard deviation of 0.2 ms?1 by correcting horizontal wind projections onto the line-of-sight direction by means of ECMWF wind data. The horizontal wind speed and direction are retrieved from lidar measurements by applying a velocity-azimuth-display-scan and a spectral accumulation technique, leading to a horizontal resolution of about 9 km along flight track and a vertical resolution of 100 m, respectively. Both vertical and horizontal wind measurements are valuable for characterizing GW properties as demonstrated by means of a flight performed on 13 December 2013 acquired during weather conditions favorable for orographic GW excitation. Wavelet power spectra of the vertical wind speed indicate that the horizontal GW wavelengths lay mainly between 10 km and 30 km and that the GW amplitude above the mountain ridge decreases with increasing altitude. Additionally, the perturbations of the horizontal wind speed are analyzed, showing horizontal wavelengths in the excitation region of 100 km to 125 km with upwind-tilted wavefronts. By means of elevation power spectra it is revealed that vertical wind power spectra are dominated by the shortwave elevation part, whereas horizontal wind perturbations are dominated by the long-wave part.